An Organometalllic NiIV Complex That Participates in Competing

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An Organometalllic NiIV Complex That Participates in Competing Transmetalation and C(sp2)−O Bond-Forming Reductive Elimination Reactions Elizabeth A. Meucci, Nicole M. Camasso, and Melanie S. Sanford* University of Michigan, Department of Chemistry, 930 North University Avenue, Ann Arbor, Michigan 48109, United States S Supporting Information *

ABSTRACT: This communication describes the synthesis of an organometallic NiIV complex bearing a labile trifluoroacetate (OTFA) ligand via the oxidation of a NiII precursor with PhI(OTFA)2. Intramolecular C(sp2)−O bond-forming reductive elimination from this NiIV complex is relatively slow, requiring 6 h at 70 °C to reach completion. In contrast, transmetalation with TMSCF3 occurs within just 1 h at room temperature to generate a NiIV−CF3 complex. These studies show that intermolecular reactions such as transmetalation can be competitive with intramolecular reductive elimination processes at NiIV centers.

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Scheme 1. Comparing Relative Rates of Transmetalation versus Reductive Elimination at NiIV

he development and study of Ni-catalyzed C−C and C− heteroatom bond-forming reactions is a rapidly emerging area in organometallic chemistry and catalysis.1 Interest in Nibased catalysts is largely driven by two factors: (1) the low cost and earth abundance of Ni relative to its second-row analogue, Pd, and (2) the ability of Ni to catalyze transformations that remain challenging with other group 10 metals.1 The vast majority of Ni-catalyzed reactions are believed to involve some combination of Ni0, NiI, NiII, and/or NiIII intermediates. However, until very recently, there have been few proposals of NiIV intermediates in catalysis.2,3 This is largely due to a lack of evidence supporting the accessibility of organometallic NiIV species with catalytically relevant oxidants.4,5 Recently, our group6 and others5,7 have demonstrated that, with an appropriate selection of ancillary ligands, a variety of organometallic NiIV complexes can be prepared via the 2electron oxidation of NiII starting materials. Furthermore, these complexes have been shown to participate in C−C and C− heteroatom bond-forming reactions that are challenging at lower valent Ni centers.6,8 In parallel with these fundamental organometallic studies of NiIV complexes, a number of recent reports have implicated NiIV intermediates in catalytic C−H functionalization reactions.3,9,10 However, these catalytic intermediates are generally believed to be short-lived and to undergo rapid reductive elimination to release functionalized products. We sought to establish whether other fundamental organometallic reactions (e.g., transmetalation) could outcompete reductive elimination at NiIV centers.11 If this were possible, it would open up a wider variety of mechanistic possibilities in NiII/IV catalytic transformations. To test this possibility, we targeted NiIV complexes of general structure A, bearing a labile X-type ligand (Scheme 1). © XXXX American Chemical Society

Complex A contains the facial tridentate tris(pyrazolyl)borate (Tp) ligand as well as a cyclometalated biphenyl ligand, which are both designed to enhance the stability of the NiIV center. As such, we anticipated that A would be isolable and would enable a detailed study of the relative rates of competing C−X bondforming reductive elimination versus transmetalation. We report herein the synthesis and reactivity of A, with X = trifluoroacetate. We demonstrate that, as predicted, this complex undergoes slow intramolecular C(sp2)−O coupling but fast intermolecular transmetalation with MCF3 reagents. The NiII starting material for these studies (1) was prepared via the sequential reaction of Ni0(PEt3)4 with potassium tris(pyrazolyl)borate (KTp) and then biphenylene (eq 1).8a,12 Received: October 26, 2016

A

DOI: 10.1021/acs.organomet.6b00810 Organometallics XXXX, XXX, XXX−XXX

Communication

Organometallics

formation of the organic product 4 in 79% yield, as determined by 19F NMR spectroscopy (eq 3). This reaction also proceeded

Complex 1 was isolated in 91% yield by washing the crude solid orange product with pentanes, and it was characterized by 1H, 13 C, and 11B NMR spectroscopy.

Hypervalent iodine oxidants have been used in the literature for the 2-electron oxidation of both Pd II and Ni I I complexes.13−15 During this oxidation process, a ligand from the iodine(III) center is typically transferred to the MIV product. As such, we selected bis(trifluoroacetoxy)iodobenzene (PhI(OTFA)2) as an oxidant to generate a NiIV complex bearing a labile trifluoroacetate ligand (2 in eq 2). The

in other solvents (e.g., MeCN, THF, DME, DMF, and dioxane) but afforded lower yields of 4 (see the Supporting Information for full details). The major detectable Ni byproduct was Tp2Ni, which was formed in 46% yield as determined by 11B NMR spectroscopy (theoretical maximum yield of Tp2Ni is 50%). The organic product 4 was isolated in 65% yield and was characterized by 1H, 13C, and 19F NMR and IR spectroscopy as well as by HRMS.17 As shown in eq 3, we hypothesize that 4 is formed via initial C(sp2)−O bond-forming reductive elimination from 2 to afford 3.18 The NiII intermediate 3 was not detected by 1H or 19F NMR spectroscopy during the reaction, suggesting that it undergoes rapid intramolecular attack by the nucleophilic σ-aryl carbon onto the carbonyl of the trifluoroacetate.19 This is presumably followed by proton transfer from adventitious water to release hemiketal 4. Notably, similar reactions of Pd,20 Ru,21 and Co22 σ-aryl species have been reported. The resulting TpNiII fragment then likely undergoes disproportionative ligand exchange to yield the observed Tp2Ni byproduct along with NiII(solvent)42+.6 The relatively slow rate of reductive elimination from 2 suggests that this complex might undergo intermolecular transmetalation with an organometallic reagent. To test this possibility, we examined transmetalation reactions between 2 and various MCF3 sources (e.g., TMSCF3, TESCF3, AgCF3) to form the NiIV−CF3 product 5. Importantly, trifluoromethyl ligands are known to stabilize high-valent Ni centers,6,23 thus increasing the likelihood of obtaining an isolable NiIV product. The reactions were conducted at room temperature to limit competing C(sp2)−O coupling. As shown in eq 4, the

treatment of 1 with PhI(OTFA)2 resulted in an instantaneous color change from yellow to brown. After purification by chromatography on silica gel, the NiIV trifluoroacetate product 2 was isolated in 50% yield as an analytically pure brown solid (eq 2).16 This diamagnetic NiIV complex was characterized by 1 H, 13C, 11B, and 19F NMR spectroscopy as well as by elemental analysis. X-ray-quality crystals were obtained by layering a methanol solution of 2 with water at room temperature. The Xray structure of 2 is shown in Figure 1a, and selected bond distances and bond angles are reported in the caption.

Figure 1. X-ray structures of NiIV complexes (a) 2 and (b) 5. Thermal ellipsoids are drawn at 50% probability. The rotational disorder in the OTFA ligand and the hydrogen atoms have been removed for clarity. For 2: bond lengths (Å) Ni1−N1 = 1.890(3), Ni1−N4 = 2.039(4), Ni1−N6 = 2.041(4), Ni1−C10 = 1.959(4), Ni1−C21 = 1.970, Ni1− O1 = 1.866(6) and bond angle (deg) C21−Ni1−C10 = 83.7(2). For 5: bond lengths (Å) Ni1−N1 = 1.958, Ni1−N4 = 2.031, Ni1−C7 = 1.956, Ni1−C13 = 1.965 and bond angle (deg) C7−Ni1−C7 = 83.17.

treatment of 2 with 5 equiv of MCF3 in the presence of a base activator afforded the transmetalation product 5 in 40− 79% yield after just 1 h, as determined by 19F NMR spectroscopy.24,25 Under the optimal conditions (5 equiv of TMSCF3 and 5 equiv of NMe4F in dioxane), 5 was obtained in 58% isolated yield after purification by chromatography on silica gel. This product was characterized by 1H, 13C, 11B, and 19 F NMR spectroscopy and elemental analysis. X-ray-quality

Complex 2 is stable in the solid state at −35 °C for more than 5 months but slowly decomposes (∼45% decomposition) in a 2,5-dimethyltetrahydrofuran solution over 8 days at room temperature. However, heating a 2,5-dimethyltetrahydrofuran solution of 2 at 70 °C for 6 h resulted in complete disappearance of the NiIV starting material and concomitant B

DOI: 10.1021/acs.organomet.6b00810 Organometallics XXXX, XXX, XXX−XXX

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crystals were obtained by slow diffusion of acetone into a methanol solution of 5, and an X-ray structure of 5 is shown in Figure 1b.26 The formation of 5 in this reaction is significant because it demonstrates that NiIV complexes can undergo intermolecular transmetalation at a rate that is at least 1 order of magnitude faster than that of reductive elimination. As such, transmetalation at NiIV centers should be considered feasible in the context of NiII/IV catalytic cycles. More broadly, this study suggests that NiIV intermediates could be sufficiently long lived to undergo other elementary organometallic reactions (e.g., C− H activation, nucleometalation, migratory insertion). Finally, we investigated the reactivity of 5 toward C(sp2)− CF3 bond-forming reductive elimination. While complex 5 was stable for 24 h at room temperature in MeCN, heating this solution to 70 °C for 12 h resulted in aryl−CF3 bond formation in low yield (27% by 19F NMR spectroscopy). We hypothesized that the addition of a ligand to this reaction could help to increase the yield by trapping and stabilizing the product. Indeed, the addition of 1.2 equiv of PMe3 to the reaction led to the formation of an isolable NiII σ-aryl product (6), which was obtained in 89% yield (eq 5).27 Complex 6 was characterized

AUTHOR INFORMATION

Corresponding Author

*E-mail for M.S.S.: [email protected]. ORCID

Melanie S. Sanford: 0000-0001-9342-9436 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation Grant CHE-1111563. We gratefully acknowledge Dr. Jeff Kampf for X-ray crystallographic analysis of complexes 2 and 5, as well as funding from NSF Grant CHE-0840456 for X-ray instrumentation. We thank James Bour for valuable discussions and suggestions.

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REFERENCES

(1) (a) Rosen, B. M.; Quasdorf, K. W.; Wilson, D. A.; Zhang, N.; Resmerita, A.-M.; Garg, N. K.; Percec, V. Chem. Rev. 2011, 111, 1346− 1416. (b) Hu, X. Chem. Sci. 2011, 2, 1867−1886. (c) Montgomery, J. Organonickel Chemistry. In Organometallics in Synthesis: Fourth Manual; Lipshutz, B. H., Ed.; Wiley: Hoboken, NJ, 2013; pp 319− 428. (d) Tasker, S. Z.; Standley, E. A.; Jamison, T. F. Nature 2014, 509, 299−309. (e) Ananikov, V. P. ACS Catal. 2015, 5, 1964−1971. (2) (a) Semmelhack, M. F.; Helquist, P. M.; Jones, L. D. J. Am. Chem. Soc. 1971, 93, 5908−5910. (b) Terao, J.; Kambe, N. Acc. Chem. Res. 2008, 41, 1545−1554. (3) (a) Castro, L. C. M.; Chatani, N. Chem. Lett. 2015, 44, 410−421. (b) Yang, X.; Shan, G.; Wang, L.; Rao, Y. Tetrahedron Lett. 2016, 57, 819−836. (c) Ruan, Z.; Lackner, S.; Ackermann, L. Angew. Chem., Int. Ed. 2016, 55, 3153−3157. (4) For seminal early studies suggesting against NiIV intermediates in Ni-catalyzed cross-coupling, see: (a) Tsou, T. T.; Kochi, J. K. J. Am. Chem. Soc. 1978, 100, 1634−1635. (b) Tsou, T. T.; Kochi, J. K. J. Am. Chem. Soc. 1979, 101, 7547−7560. (5) Klein, H.-F.; Bickelhaupt, A.; Jung, T.; Cordier, G. Organometallics 1994, 13, 2557−2559. (6) (a) Camasso, N. M.; Sanford, M. S. Science 2015, 347, 1218− 1220. (b) Bour, J. R.; Camasso, N. M.; Sanford, M. S. J. Am. Chem. Soc. 2015, 137, 8034−8037. (7) (a) Dimitrov, V.; Linden, A. Angew. Chem., Int. Ed. 2003, 42, 2631−2633. (b) Carnes, M.; Buccella, D.; Chen, J. Y. C.; Ramirez, A. P.; Turro, N. J.; Nuckolls, C.; Steigerwald, M. Angew. Chem., Int. Ed. 2009, 48, 290−294. (c) Martinez, G. E.; Ocampo, C.; Park, Y. J.; Fout, A. R. J. Am. Chem. Soc. 2016, 138, 4290−4293. (d) Schultz, J. W.; Fuchigami, K.; Zheng, B.; Rath, N. P.; Mirica, L. M. J. Am. Chem. Soc. 2016, 138, 12928−12934. (8) (a) Eisch, J. J.; Piotrowski, A. M.; Han, K. I.; Krüger, C.; Tsay, Y. H. Organometallics 1985, 4, 224−231. (b) Carmona, E.; GutiérrezPuebla, E.; Marín, J. M.; Monge, A.; Paneque, M.; Poveda, M. L.; Ruíz, C. J. Am. Chem. Soc. 1989, 111, 2883−2891. (c) Dubinina, G. G.; Brennessel, W. W.; Miller, J. L.; Vicic, D. A. Organometallics 2008, 27, 3933−3938. (d) Jover, J.; Miloserdov, F. M.; Benet-Buchholz, J.; Grushin, V. V.; Maseras, F. Organometallics 2014, 33, 6531−6543. (e) Yamamoto, T.; Abla, M.; Murakami, Y. Bull. Chem. Soc. Jpn. 2002, 75, 1997−2009. (9) For representative early examples, see: (a) Aihara, Y.; Chatani, N. J. Am. Chem. Soc. 2014, 136, 898−901. (b) Yokota, A.; Aihara, Y.; Chatani, N. J. Org. Chem. 2014, 79, 11922−11932. (c) Iyanaga, M.; Aihara, Y.; Chatani, N. J. Org. Chem. 2014, 79, 11933−11939. (10) For more recent examples, see: (a) Khan, B.; Kant, R.; Koley, D. Adv. Synth. Catal. 2016, 358, 2352−2358. (b) Aihara, Y.; Chatani, N. ACS Catal. 2016, 6, 4323−4329. (c) Uemura, T.; Yamaguchi, M.; Chatani, N. Angew. Chem., Int. Ed. 2016, 55, 3162−3165. (d) Patel, U. N.; Pandey, D. K.; Gonnade, R. G.; Punji, B. Organometallics 2016, 35, 1785−1793.

by 1H, 13C, 19F, 31P, and 11B NMR spectroscopy. Notably, a recent report from our laboratory has demonstrated related aryl−CF3 coupling reactions from TpNiIV complexes bearing noncyclometalated σ-aryl ligands.6b These previous reactions proceeded under significantly milder conditions (within 15 h at 55 °C). The lower reactivity of 5 is likely a result of the steric constraints of the cyclometalated biphenyl ligand as well as its electron-donating character, both of which are expected to stabilize the NiIV center. In summary, this communication describes the synthesis of a NiIV trifluoroacetate complex (2) via the oxidation of NiII precursor 1 with PhI(OTFA)2. Intramolecular C(sp2)−O bond-forming reductive elimination from 2 proved to be slow, which enabled competitive intermolecular transformations to occur at the NiIV center. In particular, transmetalation between 2 and TMSCF3 proceeded at room temperature to generate a NiIV−CF3 product. Overall, these studies demonstrate that NiIV complexes can participate in fast intermolecular ligand exchange/transmetalation reactions as well as in reductive elimination processes. This significantly broadens the types of catalytic cycles accessible via NiII/IV catalysis.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00810. Crystallographic data (CIF) Crystallographic data (CIF) Experimental details, optimization tables, and complete characterization data for all new compounds (PDF) Cartesian coordinates for the calculated structure (XYZ) C

DOI: 10.1021/acs.organomet.6b00810 Organometallics XXXX, XXX, XXX−XXX

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Organometallics (11) For low-temperature transmetalation between a NiIV complex and 1-norbornyllithium, see ref 7a. (12) For examples of the insertion of Ni0 complexes into biphenylene, see: (a) Beck, R.; Johnson, S. A. Chem. Commun. 2011, 47, 9233−9235. (b) Schaub, T.; Radius, U. Chem. - Eur. J. 2005, 11, 5024−5030. (c) Edelbach, B. L.; Lachicotte, R. J.; Jones, W. D. Organometallics 1999, 18, 4660−4668. (d) Bour, J. R.; Camasso, N. M.; Meucci, E. A.; Kampf, J. W.; Canty, A. J.; Sanford, M. S. J. Am. Chem. Soc. 2016, 138, 16105−16111. (13) For examples of the oxidation of PdII to PdIV with hypervalent iodine reagents, see: (a) Racowski, J. M.; Dick, A. R.; Sanford, M. S. J. Am. Chem. Soc. 2009, 131, 10974−10983. (b) Whitfield, S. R.; Sanford, M. S. J. Am. Chem. Soc. 2007, 129, 15142−15143. (c) Dick, A. R.; Kampf, J. W.; Sanford, M. S. J. Am. Chem. Soc. 2005, 127, 12790− 12791. (d) McCall, A. S.; Wang, H.; Desper, J. M.; Kraft, S. J. Am. Chem. Soc. 2011, 133, 1832−1848. (e) Lagunas, M.-C.; Gossage, R. A.; Spek, A. L.; van Koten, G. Organometallics 1998, 17, 731−741. (14) For the oxidation of NiII to NiIV with hypervalent iodine reagents, see ref 6b. (15) For oxidatively induced Ni-mediated radiofluorination promoted by iodine(III) reagents, see: Lee, E.; Hooker, J. M.; Ritter, T. J. Am. Chem. Soc. 2012, 134, 17456−17458. (16) Preliminary results show that complex 1 can also be oxidized to NiIV with PhI(OAc)2. (17) Wang, F.; Liu, Y.; Chen, F.; Qu, M.; Shi, M. Tetrahedron Lett. 2015, 56, 2393−2396. (18) A reviewer has suggested an alternative mechanism involving direct addition of NiII into the carbonyl of intermediate 3 followed by reductive elimination and protonation by adventitious water to afford 4. We do not have evidence to distinguish between the two possible pathways at this time. (19) For full experimental details and a mechanistic proposal, see the Supporting Information. (20) Saadi, J.; Bentz, C.; Redies, K.; Lentz, D.; Zimmer, R.; Reissig, H.-U. Beilstein J. Org. Chem. 2016, 12, 1236−1242. (21) (a) Seiser, T.; Cramer, N. Angew. Chem., Int. Ed. 2010, 49, 10163−10167. (b) Miura, T.; Shimada, M.; Murakami, M. Tetrahedron 2007, 63, 6131−6140. (22) Yan, J.; Yoshikai, N. ACS Catal. 2016, 6, 3738−3742. (23) (a) Zhang, C.-P.; Wang, H.; Klein, A.; Biewer, C.; Stirnat, K.; Yamaguchi, Y.; Xu, L.; Gomez-Benitez, V.; Vicic, D. A. J. Am. Chem. Soc. 2013, 135, 8141−8144. (b) Tang, F.; Rath, N. P.; Mirica, L. M. Chem. Commun. 2015, 51, 3113−3116. (c) Yu, S.; Dudkina, Y.; Wang, H.; Kholin, K. V.; Kadirov, M. K.; Budnikova, Y. H.; Vicic, D. A. Dalton Trans. 2015, 44, 19443−19446. (24) For complete details on the optimization of this reaction with respect to solvent, activator, and MCF3 source, see the Supporting Information. (25) Several other transmetalating reagents (including potassium benzyltrifluoroborate and dimethylzinc) were investigated under conditions identical with those of the TMSCF3/NMe4F reactions. NiIV intermediates were not detected by 1H or 19F NMR spectroscopy in these systems. (26) Complex 5 could also prepared via the oxidation of 1 with S(trifluoromethyl)dibenzothiophenium tetrafluoroborate. See the Supporting Information for complete experimental details. (27) In contrast, the addition of 1.2 equiv of pyridine under otherwise analogous conditions had minimal impact on the yield of this aryl−CF3 coupling reaction (25% yield with pyridine and 27% without pyridine).

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DOI: 10.1021/acs.organomet.6b00810 Organometallics XXXX, XXX, XXX−XXX